A conventional method for removing small amounts of oil from water (on the order of 5-1000 parts of oil per million parts of water) is to process the oil-contaminated water so as to form larger droplets of oil, and then allow the larger droplets to separate from the water by gravity. The larger droplets are typically formed by passing the oil-contaminated water through a coalescing medium, the processed liquid is conducted to a holding tank, where oil collected at the surface of the holding tank is drawn off, and the treated water discharged. The method is described in U.S. Pat. Nos. 2,933,191 and 3,417,015, and variations are described in U.S. Pat. Nos. 3,268,442 and 3,494,863.
One important deficiency with such gravity separators is that the coalescing units tend to become clogged. In an attempt to obtain purification down to a few parts per million, coalescing media having very fine pores have been proposed, and the result is that the coalescing media have high pressure drops and tend to clog easily, either with oil or with particulate matter entrained in the liquid being treated; see, for example, Chemical Week, Feb. 16, 1972, pp. 31 and 32.
Other methods of separating oil from water use socalled depth-bed filters and collect and store the oil within the filter. For example, U.S. Pat. No. 3,617,566, calls for forcibly passing oil-contaminated water through a vessel packed with flakes of atactic, non-crystalline polypropylene; the oil is said to collect on the flakes, and purified water is discharged from the vessel. More conventional depth-bed filters use particulates such as sand or gravel as the filter medium.
The depth-bed filters have several of the deficiencies described above. For example, a high degree of purification cannot be achieved unless the pores through the vessel are so fine that very high pressures are needed to force liquid through the packed vessel. And the needed pressures increase as oil is retained within the filter. Further, collection is generally by adsorption onto the surfaces of the particles, which is a rather limited area; and hydrodynamic forces developed as liquid is forced through the filter prevent collection of large layers of oil on the particles. To have appreciable capacity, conventional depth-bed filters must be quite large in size, and the movement of liquid through them is quite slow.
Another background teaching that has a peripheral relevance to the present invention is the prior use of a sorbing medium, that is, a medium that sorbs a liquid into itself and retains it there, to sorb one immiscible liquid from another liquid. Generally sorbing media have been used to remove a layer of oil from the surface of the water, as in U.S. Pat. No. 3,426,902, where a drum covered by a layer of polyurethane sponge is suggested for removing a layer of oil from the surface of water; or in U.S. Pat. No. 3,764,527, where a fibrous web is used to remove oil from a mixture of oil and water. Sorbing media have not generally been used to treat a liquid having a small amount of a dispersed liquid phase in a continuous liquid phase, especially where the liquid is a highly emulsified mechanical emulsion. A separator for emulsified liquids made from sorbing media would have low capacity because of the hydrodynamic forces discussed above for depth-bed filters.